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(Fig. 29) is, however, much more convenient for obtaining con-
stant temperatures up to 600°. It consists of three concentric
metal cylinders, the outer one being coated with non-conducting
material. They are so arranged that the heated air from a

movable ring burner passes be-
tween the two outer cylinders
(shown in section in the Fig.),
and descends to the bottom of
the central cylinder, into which
it has access through a ring
of circular holes. The hot air
is thoroughly mixed by this zig-
zag flow, and the temperature
is equalised. The bulb of
the displacement apparatus is
clamped in the interior cylinder,
and a thermometer is fixed be-
side it.

The vapour density of freshly
distilled aniline, b.p. 182°, may
be determined, the temperature
of the air-bath being adjusted
to about 240°. The adjustment is made by raising or lowering
the flame, or by altering the position of the movable ring-

Example.—0*1229 of aniline gave 31 c.c. at 7*5° and 750 mm.

FIG. 29.

A = 45-87.

Calculated for CCH7N ; A


The Cryoscopic or Freezing-point Method (Baoult).
—This method depends upon the fact, first demonstrated by
Raoult, and afterwards confirmed on theoretical grounds by
van't Hoff, that the original freezing-point of a given quantity
of liquid is lowered the same number of degrees by dissolving
in it different substances whose weights are proportional to
their molecular weights. This rule does not, however, apply to
salts, acids, &c., which appear to dissociate in certain solvents,
nor to substances which form molecular aggregates or associate
in solution. Supposing the freezing-point of 100 grams of a